Add missing ReturnTypes

[lrljoe]

Just an update to define some missing return types
And require PHPStan Level 6 to now pass before changes are merged.

All Submissions:

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

New Feature Submissions:

1. Does your submission pass tests and did you add any new tests needed for your feature?
2. Did you update all templates (if applicable)?
3. Did you add the relevant documentation (if applicable)?
4. Did you test locally to make sure your feature works as intended?

Changes to Core Features:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your core changes, as applicable?
• Have you successfully ran tests with your changes locally?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.45%. Comparing base (40485a0) to head (9ae3caf).
Report is 33 commits behind head on development.

Additional details and impacted files

@@                Coverage Diff                @@
##             development    #1823      +/-   ##
=================================================
+ Coverage          87.14%   88.45%   +1.31%     
- Complexity          1495     1598     +103     
=================================================
  Files                139      149      +10     
  Lines               3484     3725     +241     
=================================================
+ Hits                3036     3295     +259     
+ Misses               448      430      -18     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.